1. Field of the Invention
The present invention relates to a method of driving an ink-jet recording head used in an ink-jet recording apparatus in which recording is performed by firing ink droplets from an orifice of the ink-jet recording head toward a recording medium so that ink droplets are deposited on the surface of the recording medium, and also relates to an ink-jet recording method using this method of driving an ink-jet recording head. More particularly, the present invention relates to a method of driving an ink-jet recording head and an ink-jet recording method which can provide a high-density color image without having either significant smear (feathering) or mixture of colors (bleeding).
2. Related Background Art
Nowadays, water-based inks are most widely used in ink-jet recording apparatus since they have no problems associated with safety (toxicity) and smell. Water-based inks are commonly produced by dissolving or dispersing various water-soluble dyes or pigments into water or a liquid medium consisting of water and a water-soluble organic solvent. Furthermore, a humectant, dye dissolution promoter, mold inhibitor, or other agents are added to inks, as required,
In the ink-jet recording technique, as many as a few thousand or more ink droplets can be ejected, and thus a high-speed recording operation can be easily achieved. Another advantage of the ink-jet recording apparatus is low noise during an operation. Furthermore, the ink-jet recording technique provides a high-resolution color image on usual plain paper. These various advantages have made the ink-jet recording apparatus very popular.
Recent advancement in the technology of personal computers has led to a great reduction in their cost and also a great improvement in their performance. Furthermore, the GUI environment has also become very popular and it is now available on a standard personal computer. As a result of such advancement, a need has arisen for recording apparatus, such as a printer, having higher performance in color reproduction, image quality, durability, resolution, and operation speed. Thus, the technology of recording an image is tend to make as great an amount of coloring material as possible remain on the surface of paper so as to obtain recording dots having a sharp edge without having feathering and bleeding.
There various known techniques to suppress the feathering and the bleeding. One of such techniques is disclosed in Japanese Patent Application Laid-Open No. 58-13675 (1983) in which polyvinyl pyrolidone is added to ink to control the absorption of ink dots into paper and expansion on the paper. In another technique disclosed in Japanese Patent Application Laid-Open No. 3-172362 (1991), a specific micro emulsion is added to ink to control the absorption of ink dots into paper and expansion on the paper.
In a third known technique disclosed for example in Japanese Patent Application Laid-Open No. 62-181372 (1987) and Japanese Patent Application Laid-Open No. 1-272623 (1989), sol-gel transition in ink is used. In this technique, ink which is in a gel state at room temperature and which changes to a sol state when heated is employed. Ink droplets in a sol state are deposited on recording paper, and then ink is cooled into a gel state thereby suppressing the penetration of ink into the recording paper.
In a fourth technique which has been disclosed recently in Japanese Patent Application Laid-Open No. 6-49399 (1994), a compound having the property of heat-induced reversible gelling is added to ink so as to achieve good color reproduction and good fixing process without having significant smear in a recorded image. The resultant image formed with this ink can be stored for a long time without degradation. The patent cited here also discloses a method of driving an apparatus to record an image using such an ink. This technique is based on the phenomenon in which when a solution of a certain water-soluble polymer is heated, the solubility in water decreases, and as a result, a white precipitate is produced in the solution (the temperature at which such a white precipitate is produced is referred to as a clouding point). Typical water-soluble polymers for use the above purpose include N-isopropyl acrylamide, polyvinyl methyl ether, polyethylene oxide, and hydroxypropylcellulose. The solubility of these polymers has a negative temperature coefficient, and these polymers are separated from a solution at temperatures higher their clouding points. In the precipitated state, hydrophobic microgel is generated, which causes a reduction in viscosity of the solution. If such an ink in the precipitated state is deposited on a recording medium, the temperature the ink drops down and thus its viscosity goes back to the original high value. The above increase in the viscosity of the ink suppress the penetration of the ink into the recording medium.
On the other hand, M. Croucher et al. have pointed out the disadvantages of the conventional uniform-composition ink and has proposed ununiform-composition ink in a latex form for use in an ink-jet recording apparatus (M. D. Croucher and M. L. Hair, "Design Criteria and Future Directions in Inkjet Technology", Ind. Eng. Chem. Res. 1989, 28, pp.1712-1718).
On the other hand, U.S. Pat. No. 4,246,154 discloses (1) ink containing particles of vinyl polymer colored by a dye wherein the particles are stabilized into anionic state. U.S. Pat. No. 4,680,332 discloses (2) an ununiform-composition ink obtained by dispersing a water-insoluble polymer, which includes an oil-soluble dye and which is bonded to a nonionic stabilizing agent, into a liquid medium. Furthermore, U.S. Pat. No. 5,100,471 discloses (3) a water-based ink composed of a solvent and coloring particles each consisting of a polymer core and a silica shell bonded to a dye. This type of ink has the advantage that very vivid colors can be obtained when deposited on paper. Furthermore, this type of ink is stable at high temperatures and has high resistance to water.
In a sixth technique disclosed in Japanese Patent Application Laid-Open No. 3-240586 (1991), a non-aqueous ink consists of coloring particles dispersed into kerosene or the like wherein each coloring particle is covered with a resin which swells in the dispersing medium. This ink is said to be good in that feathering does not occur in a printed image and that nozzles via which ink is ejected are not blocked by ink.
However, in the first and second techniques described above, although penetration of ink into paper can be suppressed, ink remains on the paper for a rather long time and thus a long time is required for the ink deposited on the paper to be fixed. Another problem of these technique is bleeding.
In the case of the sol-gel transition ink according to the third technique, the ink should be stored in a proper temperature range, otherwise the ink can become soft and can flow, which will cause bleeding and smear in a printed image.
In the ink containing a compound which gels in a reversible fashion when heated according to the fourth technique, since water-soluble cellulose ether or the like is used, the viscosity of the ink increases slowly when the ink is cooled. Therefore, this type of ink is not suitable for use in a high-speed recording operation which is generally essential in an ink-jet recording apparatus in which one pixel is usually recorded in a few ten or msec. or in a shorter time. Furthermore, the ink for use in the ink-jet recording apparatus should have a low viscosity less than 20 mPa.multidot.sec. when ink is ejected. This means that it is difficult to achieve a sufficient amount of increase in the viscosity. Therefore, the increase in the viscosity.
Of three of the fifth techniques, the technique (1) in which the ink is stabilized with anions has the disadvantage that stable dispersion is possible only in a narrow pH range. Furthermore, dyes which can be employed in this technique are limited. Another problems is that ink dots deposited on paper do not expand to a sufficient extent and thus it is difficult to obtain a high enough optical density. For use in a high-speed recording operation, it is required to fix deposited dots in a short time. However, in this type of ink, as in other conventional inks, fixing of the ink occurs essentially only by evaporation and penetration and thus it is difficult to reduce the fixing time to a sufficiently low level.
On the other hand, in the case of the ink includes a dispersed water-insoluble polymer bonded to a nonionic stabilizing agent according to the technique (2), although a wider variety of dyes can be employed, fixing mechanism is also based on the evaporation and penetration of ink, and therefore it is difficult to reduce the fixing time to a sufficiently low level. Furthermore, the long fixing time can cause bleeding.
On the other hand, although the dispersion ink having the polymer core/silica shell structure according to the technique (3) is excellent in that the dye is dispersed in a stable fashion, this type of ink does not have any special means for aggregating a coloring material when the ink is deposited on paper. As a result, the ink cannot provide a high enough optical density. Furthermore, the ink deposited on paper is fixed only by evaporation and penetration, the fixing time is rather long and bleeding occurs.
The problem common for all three techniques described above is that adhesion of coloring particles to the surface of paper is not taken into consideration and thus the recorded image is poor in resistance to rubbing.
In the sixth technique, since kerosene is used as the dispersion medium, the ink has problems of smell and toxicity.
Now, transfer of ink onto paper will be discussed. It is known that the process of transferring liquid onto paper can be represented by Lucas-Washburn's formula. When the liquid is water, the Lucas-Washburn's formula becomes as follows: ##EQU2## where V denotes the amount of liquid transferred onto paper, V.sub.r the constant representing the roughness of paper, K.sub.a the absorption coefficient, T the transfer time, and T.sub.w the wetting start time. In the above formula, the K.sub.a depends on the properties of paper and ink and can be represented as follows: ##EQU3## where r denotes the radius of capillaries, .gamma. the surface tension of the liquid, .theta. the angle of contact between the liquid and paper, and .eta. the viscosity of the liquid.
It can be seen from formula (1) that in order for the coloring material to remain on the surface of paper, it is required that the penetration of the liquid should be as slow as possible, that is, Ka should be as small as possible (so that evaporation can occur during the slow penetration process). To obtain a small value of K.sub.a, it required that the ink should be small enough in surface tension and should be large enough in the viscosity and the angle of contact. However, for use in the ink-jet recording operation, the ink should have particular properties, and this requirement makes it difficult to adjust K.sub.a to a desired value.
On the other hand, in the case where the liquid is a non-aqueous solvent such as ethanol or the like, the wetting time T.sub.w is small enough to be neglected in formula (1). This results in a reduction in the fixing time. However, K.sub.a becomes great, and quick penetration occurs. As a result, a recorded image will have a great amount of feathering. In formula (2), the term cos.theta. depends on the combination of ink and paper. In other words, although a certain kind of paper may result in a desirable value of cos.theta., another kind of paper may result in an undesirable value of cos.theta.. Thus the cos.theta. is sensitive to the kind of recording paper used. This is undesirable in applications of ink-jet recording apparatus.
The problems describes above can also occur in conventional inks of the coloring-material dispersion type as long as the mechanism of fixing a recorded image is based only on the penetration and evaporation of ink.